large recesses. New roller gate installations are not being used nowadays
partly because large units of this type are very vulnerable to single point
failure.
6.2.7 Fabric gates
Inflatable rubber or fabric gates can be pressurized by air, water or both.
They usually have an inner shell and an outer casing, and can be used to
close very large spans (over 100 m) with heads up to 6 m. The strength of
fabric used exceeds 100 kN m^1.
The main advantages of fabric gates are low cost, low weight,
absence of lifting mechanism, little need for maintenance, acceptance of
side slope (at river banks), and ease of installation; the disadvantage is that
they can easily be damaged and may have a limited life.
6.2.8 Overspill fusegates
Fusegates for installation on previously ungated spillways use the principle
of fuse plugs (Section 4.7.7) without the disadvantages particularly of the
sudden large increase in discharge and the loss of the plug.
Designed and used first in France under the name Hydroplus
fusegate (in 1991 Lussas irrigation dam with 10 gates 2.15 m high and 3.5 m
wide) (Lemperière, 1992) their height can be up to 75% of the free spill-
way head (saving of storage) and they can pass moderate floods as a spill
over their labyrinth-shaped top. For higher floods the independent free-
standing units turn about a downstream sill, if a bottom chamber filled
through an inlet well generates an uplift pressure beneath the unit. After
the flood has receded the overturned units can be reinstalled on the crest
or, if damaged, replaced by new ones; they remain on the spillway chute
below the crest only for velocities lower than 3 m s^1 , otherwise being
washed away. Should the need for their recovery or replacement occur fre-
quently, the fusegates may become uneconomical.
Since their first use much bigger fusegates have been installed. E.g.
during the reconstruction of the Terminus dam in California (to pass a
revised PMF) six concrete units each 11.7 m wide, 6.5 m high with 13 m
long crest have been used. The opening (overturning) of the units is stag-
gered with the first tipping at water levels 7.8 m and the last at 9.3 m above
the dam crest (Kocahan 2004).
In Russia fusegates 1.75 m high and 3.15 m wide as well as a proto-
type of a new folding fusegate type gate were successfully tested next to a
labyrinth spillway in very harsh winter conditions (Rodionov et al., 2006).
274 GATES AND VALVES
